Maybe that's his problem. After he lies there stuck for a few minutes, he CAN'T get up for the simple reason that he can't breathe. The question is, why can't he get up right away, as most chickens would? Or even, how and why is he ending up on his back, as most chickens would not? When dust bathing, most chickens remain on one side or another, not their backs. Are his spurs too long? I'd check for weaknesses or injuries to his legs.This is really strange! I'd get him to the vet to be honest if you can afford it/want to. If he is wriggling he probably wants to get up but can't. As I said earlier, the weight of the organs on the solid lungs can cause breathing issues.